U.S. Pat. No. 6,782,122 describes apparatus for measuring height of a surface of liquid in a container utilizing a light source and camera pair disposed above the liquid surface in a predetermined spatial relationship to each other and the container. The light source illuminates the liquid surface with a light pattern and the camera captures the area image of the light pattern in a form of discrete picture elements (pixels) that are processed in an image processor to determine the height of liquid in the container. In another embodiment, the pixels of the area image are further processed to determine attitude of the liquid surface so that the processor may determine the quantity of liquid in the container based on the determined height and attitude of the liquid surface and container geometry data stored in a memory thereof.
Another liquid level detection method is described in US2010/0322462. The method includes capturing an image of a liquid surface, a structural surface, and graduation markings provided on the structural surface using an image-capturing device to thereby obtain an initial image. Subsequently, the initial image is processed so as to generate a processed image, and a level reference value of the liquid surface is obtained from the processed image. The level reference value represents a height of the liquid surface in terms of inherent characteristics of the processed image. Lastly, a liquid level of the liquid surface is calculated based on a relative proportional relation among the level reference value, an overall height of the processed image in terms of the inherent characteristics of the processed image, and dimensions of any one of the initial and processed images relative to the graduation markings.
A method of remote monitoring of fixed structures is described in US2008/0282817. In one embodiment, an optical system is designed to project scales on the inner surface of three walls of a housing, and obtain images of the wail(s) at the projected locations of the scales. This information is used to derive the level of fluid in the tank.